Many members for automobiles have a complicated shape, and need to be made of a material having excellent elongation, stretch flangeability, and bendability, which are indices of formability. When the strength is increased to a TS of the 900 MPa class or more, a very expensive rare element such as Ti, Nb, V, Cu, Ni, Cr, or Mo is sometimes intentionally added to achieve high strength. Also, techniques disclosed in Patent Literature 1 and Patent Literature 2 are known as a technique in which high elongation is achieved while high strength is achieved by generating a retained austenite phase in a microstructure and using a TRIP effect due to strain induced transformation.
Patent Literature 1 proposes a technique concerning a steel sheet having excellent stretch flangeability. The steel sheet has a particular composition, and includes a primary-phase microstructure containing tempered martensite or tempered bainite, or also ferrite at a respective particular area fraction and a second-phase microstructure including retained austenite wherein the fraction of lath retained austenite in the retained austenite is 70% or more. Patent Literature 1 discloses a method for producing such a steel sheet. In the method, when tempered martensite or tempered bainite is a primary phase, a steel sheet subjected to a hot-rolling step and a cold-rolling step is rapidly cooled from an A3 transformation temperature or more (γ region) to an Ms temperature or less to obtain a martensite phase (quenched martensite phase) or is rapidly cooled from an A3 transformation temperature or more (γ region) to an Ms temperature or more and a Bs temperature or less to obtain a bainite phase (quenched bainite phase), heated and held at a temperature of an A1 transformation temperature or more (about 700° C. or more) and an A3 transformation temperature or less, and then cooled to a predetermined temperature and held for a predetermined time to temper the primary-phase microstructure while at the same time a desired second-phase microstructure is obtained. When a mixed microstructure of tempered martensite and ferrite (α) or a mixed microstructure of tempered bainite and ferrite (α) is a primary phase, a steel sheet subjected to a hot-rolling step and a cold-rolling step is rapidly cooled from an A1 transformation temperature or more and an A3 transformation temperature or less or an A3 transformation temperature or more to an Ms temperature or less to obtain a mixed microstructure (quenched martensite phase+α) of a martensite phase and ferrite or is rapidly cooled to an Ms temperature or more and a Bs temperature or less to obtain a bainite phase (quenched bainite phase+α), heated and held at a temperature of an A1 transformation temperature or more (about 700° C. or more) and an A3 transformation temperature or less, and then cooled to a predetermined temperature and held for a predetermined time.
Patent Literature 2 discloses a technique concerning high-strength steel sheet having excellent stretch flangeability. The steel sheet has a particular composition including 2 to 20% of a retained austenite phase and, as a main phase, “a bainitic-ferrite (BF) phase having high dislocation density, which is distinct from a bainite phase”. This technique proposes that the stretch flangeability of TRIP steel sheets including BF as a primary phase is improved by controlling the fraction of fine retained austenite to be 60% or more.
Patent Literature 3 discloses a technique concerning a high-strength thin steel sheet having excellent elongation and hole expandability (stretch flangeability). The steel sheet has a metal microstructure containing 3 to 30% of a retained austenite phase and mainly containing ferrite, bainite, or tempered martensite. This technique proposes that the concentration gradient of C in an austenite phase is controlled by controlling the cooling conditions after the concentration of C into the austenite phase is facilitated by bainite transformation or tempering of martensite, the stability of the retained austenite phase is increased, and thus the TRIP effect is effectively produced without degrading the hole expandability.